1. Field of the Invention
The present invention relates to ultrafine metal powder slurry with high dispersibility, which may be used as, for example, conductive paste fillers or internal electrodes of multilayer ceramic capacitors.
2. Description of the Related Art
Ultrafine metal powders for use in internal electrodes of multilayer ceramic capacitors are high-purity metal powders having a mean particle diameter of, for example, 0.1 to 1.0 μm and having a substantially spherical shape. Such an ultrafine metal powder is mixed with a binder such as an organic resin and thereby yields a paste. A thin film of the paste is applied to a ceramic green sheet by, for example, screen printing to form an internal electrode layer. Several hundreds of plies of the ceramic green sheet with the applied paste thin film are stacked and thereby yield a laminated composite. The laminated composite is subjected to a binder burn out process, a sintering process, and a firing process and thereby yields a multilayer ceramic capacitor. The mean particle size used herein means a mean surface diameter (D3) in a number-based particle size distribution.
To yield downsized and high-capacity multilayer ceramic capacitors, several hundreds to thousand plies of a ceramic green sheet with an internal electrode layer must be stacked. To accomplish this technology, the thickness of the internal electrode layer must be reduced from 3 μm in conventional equivalents to 1.5 μm or less.
If the ultrafine metal powder has poor dispersibility and thereby yields aggregates such as clumps, the aggregates penetrate the ceramic sheet layer and thereby lead to defective units with short-circuit of electrodes. Even if there are no penetrating aggregates, distances between the electrodes decrease to cause local current crowding to thereby shorten the life of the multilayer ceramic capacitor.
Such ultrafine metal powders have conventionally been produced by chemical vapor deposition (CVD), in which the prepared metal powder is subjected to a wet washing process in water for purification to yield a metal-water slurry, and the metal-water slurry is subjected to a drying process to yield a dried powder. These processes inevitably cause aggregated particles. Accordingly, the dried powder is not sufficiently dispersed in an organic solvent in a paste preparation process step 20 shown in FIG. 2. Accordingly, the dried powder is further subjected to a dispersing process step 25 including several dispersion treatments such as ball mill dispersion, ultrasonic dispersion, and roll mill dispersion in combination. However, the ultrafine metal powder having been subjected to the drying process tends to aggregate and has poor dispersibility also in the dispersing process step 25. These dispersion processes using a conventional dried powder can only yield a paste comprising an ultrafine metal powder in a content of at most about 50% by mass.
FIG. 2 shows the conventional process 20 for the making paste preparation of an ultrafine metal powder. In a metal powder purification process step 21, the metal powder is purified by removing a remainder material metal chloride and thereby yields a metal-water slurry (metal-water mixture). The metal-water slurry is dried in a metal powder drying process step 22 and thereby yields a product ultrafine metal powder, i.e., a dried powder 23. In the drying process step 22, aggregated particles are formed due to dry aggregation caused by liquid bridging force and van der Waals force acting among particles.
Aggregation also occurs in the subsequent process step 24 for dispersing the dried powder in an organic solvent. In the drying process step 22, metal hydroxides are formed on a surface of the ultrafine metal powder and thereby the resulting ultrafine metal powder has insufficient wettability (lipophilic property) to organic solvents. Accordingly, particles of the ultrafine metal powder having poor wettability to the organic solvents aggregate with one another in the dispersing process step 24 for dispersing the dried powder in an organic solvent.
The dried powder 23 is supplied to a user. On the user side, the dried powder 23 is subjected to the dispersing process step 24 in an organic solvent, is further treated via a viscosity adjustment process step 27 and thereby yields a paste. To disaggregate aggregated particles formed in the drying process step 22 and in the dispersing process step 24, the process further requires complicated treatments including dispersing process step 25 including, for example, ball mill dispersion, ultrasonic dispersion, and roll mill dispersion in combination and a filtrating process step 26. The process thereby requires much time and effort. Accordingly, although the dried powder 23 of the ultrafine metal powder as a product is required to include no aggregated particles and have high dispersibility, no conventional ultrafine metal powder slurry satisfy these requirements.
In addition, when the dried powder 23 supplied to the user is taken out from a package and is treated in the air for the above complicated treatments, an operator exposes oneself to jeopardy to inhale metal dust.